A humidity measurement device is one of the sensors that are attached to intake systems of internal combustion engines of automobiles for improved fuel economy and improved environmental performance. The humidity measurement device includes a measurement element of relative humidity, a measurement element of temperature provided near the measurement element of relative humidity, a heating element provided near the measurement element of relative humidity, and a circuit part that controls the measurement element of relative humidity, the measurement element of temperature, and the heating element. The humidity measurement device uses outputs of the measurement element of relative humidity and the measurement element of temperature, to calculate a specific humidity and externally transmits a signal corresponding to the specific humidity.
Here, there is a problem that gas to be measured flowing in an air intake system contains contaminants, such as dust, which could not be trapped by an air cleaner. The contaminants adhere to the measurement element of relative humidity, which degrades a measurement accuracy of relative humidity and accordingly degrades a calculation accuracy of a specific humidity in the humidity measurement device. PTL 1 is a conventional technique for recovering from the accuracy-degraded state caused by adhesion of contaminants to the measurement element of relative humidity.
PTL 1 discloses a technique that determines, under an environment that is not humidified or dehumidified, without a pressure change, a difference between dew-point temperatures of before heating and of during the heating, to self-diagnose whether a deterioration occurs to a degree requiring cleaning.
According to PTL 1, a degradation-diagnosis processing may be started in response to a starting instruction from a user, and may be started on a regular basis, and when the self-diagnosis determines that a humidity element is deteriorated, heating cleaning is performed to remove a component, in an atmosphere, that causes the deterioration of the humidity element, so that the humidity element can be recovered from the deterioration.